Radio navigation system



Dec. 9, 1947.

G. D. HULST, JR., ETAL.

RADIO NAVIGATION SYSTEM Sheets-Sheet 2 Dec- 9, 1947- G. D. HuLsT, JR., ETAL RADIO NAVIGATION SYSTEM,

Filed Dec. 14, 1944 Dec# 9, 1947- G. D. HULST, JR.,A ETAL RADIO NAVIGATION SYSTEM Filed Deo@ 14, 1944 5 sheets-sheet 5 De@ 9,' 1947- G. D. HULST, JR., ETAL .2,432,158

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Dec. 9, 1947.

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Patented Dec. 9, 1,947

2,432,158 y moro NAVIGATION SYSTEM George D. Hulst, Jr., Upper Montclair, N. J., and

Earl Schoenfeld, Mamaroneck, and Garrard Mountjoy, Manhasset, N. Y., assignors to Radio Corporation of America, a corporation of Dela- ,Ware

Application December 14, 1944, Serial No. 568,084

(Cl. Z50-11) 6, Claims. 1

Our invention relates to radio navigation systems and particularly to systems of the type utilizing the time difference in the propagation of radio pulses from synchronized ground stations.

Navigation systems of this type emplOy pairs of synchronized ground stations that transmit radio pulses having at the instant oi radiation a fixed time relation to each other. Each pair of ground stations preferably transmits pulses at its individual assigned repetition rate for the purpose of station selection. The pulses are radiated to receiving equipment located on the aircraft or ship whose position is to be determined. By

means of the receiving equipment, the operator on the craft determines the time difference between the pulses from the two transmitter stations of one pair as they arrive at the receiver. Since the radio pulses travel from the ground transmitters to the receiver at a known propagation rate (i. e., at the velocity of light), it is known that the position of the craft is at some point on a line corresponding to the time difference reading. By obtaining the time diierence reading from a second pair of ground stations, a second line corresponding to the second time difference reading is obtained, and the intersect point of the two lines is the position of the craft. Special maps having time difference lines printed thereon for the several pairs of ground stations are provided for use with the'navigation system.

In order to measure the time difference in the arrival of successive pulses from a pair of ground stations, timing marker pulses that have a known time interval between them are generated. Also, pulses having a denite time relation to the time marker pulses are generated for the purpose of driving or synchronizing cathode ray deecting circuits. These deecting circuits produce cathode ray sweep traces on which the marker pulses and/or the received ground station pulses appear.

For the purpose of selecting a particular pair of ground stations, the operator selects a particular pulse repetition rate for the driving or synchronizing pulses corresponding to the repetition period of the pulses transmitted from said pair whereby the deecting circuits may be synchronized with the received pulses from the selected pair of ground stations. Thus a particular pair of ground stations is selected at the receiver apparatus byv turning a station selection switch to the position indicated on the receiver panel for obtaining sweep synchronizing pulses having the same repetition period as that of the pulses being transmitted from the selected pair of ground. stations. Now the received pulses 2 from the selected pair of ground stations can be made to appear stationary on the cathode ray sweep or trace whereas those received from the other pairs of ground stations Will move rapidly along the same trace.

In operation, the pulses from the two transmitter stations of a selected pair of stations (which pulses will be referred to as A and B pulses, respectively) are made to appear on two cathode ray traces and are brought into alignment or coincidence by moving one of them along its cathode ray sweep trace, this being done by adjusting the start of the cathode ray sweep. It is then possible to measure the time displacement of the sweep required for pulse alignment. This is done by counting certain timing markers appearing on the cathode ray traces and thus the desired time difference between pulses is determined. The present application describes an improved method of and system for thus determining the desired time difference.

An object of the present invention is to provide an improved method of and means for determining the time difference between electrical pulses.

A further object of the invention is to provide improved receiving equipment for a radio navigation system of the type utilizing the propagation of radio pulses from pairs of synchronized ground stations.

A still further object of the invention is to provide an improved method of and means for indicating the time difference between radio pulses transmitted from synchronized ground stations.

The invention will be better understood from the `following description taken in connection with the accompanying drawing in which Figure l is a block diagram of navigation receiving apparatus designed in accordance with one embodiment of the invention,

Figure 1a is a block diagram representing one pair of ground radio transmitter stations of the navigation system which transmit A and B pulses, respectively.

Figure 2 is a block and circuit diagram of the pulse generating unit shown in Fig. 1,

Figure 3 is a view of the screen end of the cathode' ray indicator tube included in the apparatus of Fig. 1 showing the cathode ray sweep traces with the received pulses A and B in alignment and with the timing marker pulses indicating the time interval between the received A and B pulses.

Figure 4 is a view showing the relation of the cathode-ray traces to the horizontal deflecting waves,

Figure is a group of graphs which are referred In the several figures, similar parts are indi:-`

cated by similar reference characters.

The pulse generator um't.

In Fig. 1, the pulse generatingl circuit for producing the timing marker pulses-and'for producing the controlling or synchronizing pulses that control the cathode-ray deflection is shown in block diagram at the top of: the, figure; It is shown in detail in Fig. 2. Referring to Figs. 1 and 2, the pulse generator comprises a crystal oscillator I that produces a sine wave voltage of stablefrequency whichI in. the example illustratedis- 100kilocycles--per second,J the, repetition period. being limicro-seconds. The frequency ofx the crystaloscillator outputmay be increased or decreased slightly by a manual adjustment as indicated at the control knob fory obtaining a right or left. drift of. areceivedpulse on a. cathoderay sweep trace.

The crystal oscillator1 |0'drivesa blocking oscillator I2. or. the like.. to produce periodic pulses which, in the present example, also recur at the rate of100 k. c. per second.Y Therepetition period o-r time interval. between; successivey pulses is, therefore, micro-seconds..

The frequencyv ofthe 10. p.. s. pulsesis divided by five by means of; a suitablefrequency divider I3 such as a second blocking; oscillator to produce 50 p.. s. pulses. While specificA values are being given for the severalfrequency division steps, the invention is notlimitedlto these particular values.

The 50 n. s. pulses are applied through a lead to a frequency divider I6; of the counter type described in White latent'` 2,113,011. It divides the frequency by, two-togproduce 100 e. s. pulses. Also, an additionalcircuit is provided so that the divider I6,y may be made,to.lose;a,count for the purpose of obtaining` a: different` selected pulse repetition period.4

The divider I6 comprises a counter circuit' portion including'an input orA bucket capacitor |1, a pair of diodes, I8: and |9-a storagef capacitor 2| and ablockingoscillator portion122. In addition,.it includes-a pairfofdiodes 23.and 26 associated with thestorage capacitor 2| for the purpose of making the divider. |6..-lose a count upon the applicationof a.pulse froma conductor 26 as will be explained hereinafter. The blocking oscillator 22r comprisesa vacuum tube 21; a transformer 28 couplingM the platev circuit tothe grid circuit and a4 cathode biasing resistor 20 which is bypassed by a capacitor 3|. A- transformer 32 supplies-.the 100 n. szpulses fromthe divider I6 to a frequency divider 33 which also isofithe type which may be made t'o lose-afcount'.

The frequency divider: I6 operatesasA follows: Eachi of the 50` ,u s. pulses, ofv positive polarity from the lead |f|` puts apredeterminedcharge on thev comparatively largecapacity storage capacitory 2z| as aresult` of: a.. pulse; of current through the comparatively, small bucket capacitor l1 and throughgthediode: I9; the capacityl of the capacitor llfbeingsmalkenough so thatcapacitor |.1freceives fullicharge before thetermination of an applied pulse. At the end of this current pulse, the capacitor: |12 is-.dischargedztogroundpotential through the diode I8. The next 50 fr. s. pulse puts an additional current pulse into capacitor 2|, this raising the voltage across capacitor 2| suiciently to trigger the blocking oscillator 22 whereby a pulse is produced across the transformer 28 as is Well understood in the art. The pulse thus produced is applied to the divider 33 with positive polarity. At the same time the blocking oscillator 22 discharges the capacitor 2| to bring, it back to ground potential.

The frequency divider 33 divides the frequency by ve to produce 500 p.. s. pulses. It includes a counter portion comprising a bucket capacitor 36, av pair ofl diodes 31 and 38, and a storage capacitor 39; Italso; includes a blocking oscillator portion 4| comprising a vacuum tube 42, a feedback transformer 13, a biasing resistor 44 and a bypass capacitor 46.

As in. the preceding divider I6, there is provided in the divider 33 a pair of diodes 41 and 48 for subtracting. counts. In the divider 33, however, the application-.of a pulse from a conductor awill subtract one; twoor three counts depending upon they position; of:v the station selection switch.

The 500 n. s. pulses aresupplied over a conductor 5| to a frequency divider 52.that divides by two to produce 1000 It. s. pulses. The divider 52 is similar to thedivider |6 with the count subtracting diodes omitted.

The 1000 p.. s, pulses are supplied toa frequency divider 5.6 that divides by ve to produce 5000 it. s. pulses which, in turn, are supplied to a frequency divider 59 that divides by four to produce 20,000 n. s. pulses. The dividers 56 and 59 are similar to the divider 52 except for the difference in circuit constants.

The 20,000'11.. s.,pul'ses may be passed through a clipping circuit 63" and supplies over a conductor 6| to a square wave generator 65 (Fig. 1) such as an Eccles-Jordan oscillator for obtaining a square wave having a repetition period of 40,000 p. s. From this square wave are obtained, by means of suitable wave shapingand delay circuits described hereinafter, the desired driving or synchronizing pulses for the horizontal deflection.

The 20,000 it. s. pulses are alsosupplied over a conductor 62' and' through a bucket capacitor 630i the firstv count subtraction circuit to a station selection switch 64; they are also supplied to the second countsubtraction circuit through a coupling or blockingcapacitor 66,Y of large capacity to a second stationselection switch 61 which is ganged with the switch 64 as. indicated by the broken line 68thetWo-switchesbeing operated by a knob 65.

At` the switch 64, `alternate switchcontact points I are connected to the feedback conductor 26 whereby at these ,switch pointpositions the 20,000 it. s. pulses are fed back tothe dividerY I6 to subtract counts. It mayV be .desirable because of distributed or stray leakage in-the switch 64 or capacitors 63 to connect its switch arm to ground through a 1 megohmresistor 55.to permit charges to leak off,

At the switch 61,.the;1ast six` switch contact points are connected in.pairs, the three-pairs of contact points` No. 2-No. 3, No. Ll--No. 5, and

No. 6-No. 7 being connected through bucketl capacitors 1|, 12 and13; respectively, to the feedback. conductor, 43.- vvhich leads to the second count subtraction-circuit. Thus, with switch 61 in-any one of the-last six positions, 20,00() p. s. pulses are applied to'thefdivider. 33 to subtract counts.

Before discussing in detail the operation of the count subtracting circuits for station selection, it may be noted that the desired timing marker pulses are obtained at various points along the frequency divider circuit. In` the present system, the c. s. pulses are supplied from the blocking oscillator I2 to an output lead 8|. 'I'he 100 p. s. pulses are supplied over a conductor 84 to an output lead 81. The 1000 li. s. pulses and 20,000 fr. s. pulses are supplied to output leads 88 and 89. The marker leads 8|, 81, 88 and 89 supply the 10 n. s. pulses, the 100 c. s. pulses, the 1000 fr. s. pulses and the 20,000 n. s. pulses to a mixer tube or circuit |29 (Fig. 1) and from the mixer |29 to a vertical delecting plate of a cathode ray tube II6 as described hereinafter. The cathode ray of the tube II6 is deflected horizontally by a deflecting` wave that is in synchronism with the 40,000 c. s. square wave from the Eccles-Jordan oscillator 55 (Fig. 1). It is evident that the 40,000 p.. s. horizontal deection cycle has a xed time relation to the ltiming marker pulses.

`Count subtraction ,for station selectionY Referring now more particularly to the feature of subtracting counts for the purpose of station selection, specic pulse repetition rates for a plurality of pairs of ground transmitter stations will be referred to vby way of example to aid in explaining the operation.

It will be assumed that the first pair of ground stations transmit the A pulses with a repetition period of 40,000 p. s. and transmit the B pulses withV a.like repetition period; that the second pair of ground stations transmit A and B pulses having a repetition period of 39,900 p.' s.; that the third pair transmits 39,800 n. s. pulses; that the fourth pair transmits 39,700 li. s. pulses, etc. It is-V apparent that for station selection at the receiving apparatus, the operator must be able to select corresponding repetition periods for the output of the square wave generator 65 which controls the cathode ray deflection cycle; namely, periods of 40,000 li. s.; 39,900 n. s.; 39,800 p.. s.; 39,700 fr. s.; 39,600 p.. s.; etc.

It will be noted that the several repetition periods dii-ier from each other by 100 It. s. or by integral multiples thereof, and that this corresponds to repetition period differences of 50 p. s. or integral multiples thereof at the output of the frequency divider chain, i. e., at the input of the clipper 60. Therefore, the desired repetition period can be obtained by shortening the 20,000 c. s. period by 50 p. s., by 100 p.. s., by 150 fr. s., etc.

For example, to obtain the 39,900 fr. s. repetition period the switches 64 and 61 are moved to the No. l'switch contact points. At this switch position the 20,000 ,L s. pulses from the lead 62 are fed back by way' of the bucket capacitor 63, the switch 64 and the conductor 26 to the frequency divider I6 only. Upon the occurrence of a 20,000 n. s. pulse, it produces a pulse of current through the bucket capacitor 63 and through the diode 23 to add a charge to the storage capaoitor 2i. At the end of the pulse, the capacitor 63 discharges through the diode 24 to its original potential. By properly selectingthe capacity value of the bucket capacitor 63, the added charge is made equal to the charge which is added to the capacitor 2l by a single 50 1.a. s. pulse. Thus, the 20,000 fr. s. pulse causes the blocking oscillator 22 to fire one pulse earlier or 50 p. s. sooner than it normally would whereby the desired repetition period of 19,950 p.. s. at the clipper 6U or 39,900 c. s. at the output of the E--J oscillator 65 is obtained. It may be noted that, inthe example given, each time a 20,000 fr.' s. pulse occurs, the divider I 6 divides by one instead of by two.

, To obtain the 39,800 p.. s. repetition period, the switches 64 and 61 are moved to position No. 2. Now the 20,000 it. s. pulses are applied through the bucket capacitor 'II to the divider 33 and upon the occurrence of a 20,000 fr. s. pulse it applies a charge to the capacitor 39 through the diode 48. At the end of the pulse the capacitor 1I discharges through the diode 41 to its original potential. The capacitor 1I is given a capacity value such that this charge applied by the 20,000 ,u. s. pulse is equal to the charge applied by a single n. s. pulse. Thus, upon the occurrence of a 20,000 .Vs. pulse the blocking oscillator 4I fires one pulse early or 100 c. s. sooner than it normally would whereby the desired repetition period of 19,900 p.. s. is obtained at the clipper 60 and a repetition period of 39,800 n. s. is obtained at the output of the E-J oscillator 65. It may be noted that in the example given, the divider 33 divides by four instead of by ve upon the occurrence of each 20,000 ,u. s. pulse.

To obtain the 39,700 p. s. repetition period, the switches 61 and 61 are moved to the No. 3 position, this being the switch position shown in the drawing. Now the 20,000 it. s. pulses are applied to both the divider I6 and the divider 33 through the switches 64 and 61 whereby both dividers lose a count. Specifically, the blocking oscillators 22 and 4I of dividers I6 and 33 re 50 p. s. and 100 p. s. early, respectively, or a total of 150 p. s. early. Thus, the desired repetition period of 2 l9,850 n. s. or 39,700 p. s. is obtained at the E-J oscillator output.

To obtain the 39,600 fr. s. repetition period, the switches 64 and 61 are moved to thev No. 4 position. Again the 20,000 p.. s. pulses are applied to the divider 33 only, but this time through the capacitor 12 which has a capacity value such that a 20,000 fr. s. pulse causes the divider 33 to lose two counts, i. e., to trigger 200 fr. s. early. Thus, the desired period of 2 l9,800 n. s. or 39,600 c. s. is obtained at the E-J oscillator.

At the No. 5 switch position, the divider I6 again triggers 50 l. s. early and the divider 33 triggers 200 p.. s. early, or a total of 250 n. s. forV the two dividers. Thus, the repetition period is 19,750 l. s. at the input to clipper 69 or 39,500 p. s. at the output of the E-J oscillator 65.

At the No. 6 switch position, only the divider 33 receives the 20,000 p.. s. pulses. These pulses are applied through the capacitor 13 which is adjusted to make the divider 33 lose three counts. Thus, it triggers 300 fr. s. early to give a repetition period of 2X 19,700 c. s. or 39,400 p.. s. at the E-J oscillator output.

At the No. 7 switch position, both of the dividers I6 and 33 lose counts, divider I6 triggering 50 n. s. early and divider 33 triggering 300 ,11. s. early, or a total of 350 ,u. s. whereby the repetition period is 19,650 u. s. at the clipper 60 or 39,300 p.. s. at the E-J oscillator output.

It may be preferred to employ a different group of repetition periods than the group of 40,000 Il. s 39,900 y. s., etc.. assumed above. By making the iinal divider stage 59 divide by three, for example,

7 group fof repetition :periods of K501,000 n. is., 149;900 y.. s., etc.

In order to obtain a 'more rapidright 'fdr'.ift of theA and B pulses'in the Ypreliminary'steps fof obtaining a time difference readinggitmaybe'desirable to provide 'a capacitor 95 that may-be'connectedby a switch-9B tothe coupling capacitor' so that by closing the switch Sii-additional counts will be lost by the divider '33. Thus, the Aand -B pulses may be drifted toward the right .by `closing the switch 96. When the switchQG isopened the A and `B pulses stop drifting and again are sta tionary,

The pulse generator an'd'station selection circuit described in the foregoing pages is the same as that described in application Serial No. 552,146, filed August 31, 1944, in the nameofEarlSchoenfeld, and entitled Timing vmarker and station selection apparatus.

VCathode-ray trace and timing marker presentation Before describing that portion of thereceiving apparatus of Fig. 1 to which the timing'marker and control pulses from the pulse generator unit are applied, reference will be made'to Fig. 3'showing the appearance of the cathode ray pattern from which the time interval between the A Aand B .pulses from a pair of ground y"stations is determined. It will be noted that there are two cathode ray sweep traces, ab and cd which have their .left-hand portions expanded.

The graphs G and J of Fig. Y vshow the wave shapes of the horizontal and 'vertical deiiecting waves, respectively, for obtaining the above-described cathode ray sweep. 'The starting time t of the second saw-tooth wave of horizontal deecting wave G may be adjusted by'adjusting a multivibrator itil by a knobl 102' as will be explained hereinafter.

Referringito Figs. 3, 4-and 5, the received pulses A and B from the selected pair of stations are caused to appear on the first and second traces, respectively, at their expanded ends. This-isaecomplished by first making them stay stationary on the two sweeps by making a crystal foscillator frequency adjustment at the knob H inthe event that there is a slight drift of these A'and'rB pulses. The A and B pulses-are now brought into'the'position of alignment or coincidence as shownin Figs. 3, 4 and`5 by the following procedure. By-ad-justment of the crystal oscillator frequency at the knob Il and/or by moving the station selection switch knob 55 to obtain adiiferent'pulse repetition rate, the pulse Ais driftedont'o the-expanded end of the trace ab'. Next the pulse B isbrought onto 'the expanded end of the adjustable 'trace'cd and is brought into coincidence -with the pulse A. In order to bring the pulse B into coincidence with the pulse A, the starting time it fofith'e :second sweep of Athe horizontal deflecting wave G (Figs. 4 and) isadjusted kby adjustirrg'the multi'- vibrator Ill at the knob HB2', the circuit .toraccomplishing this being described hereinafter.

A comparison ofthe A. and .B pulses as shown vin Fig. 5 with the horizontal deflecting wave 'G `of `the same figure will show that the condition of .coincidence of 'the Y.pulses .A land .B has been :illustrated, both pulses falling on the expanded ends of successive traces and occurring at y-eofual time intervals from the starts of the traces. It will be understood that while the pulses A :and B and their' vcorresponding traces appear alternately 'on the Vcathode ray tube screen, :they appearto the eye to occur simultaneously because o'f `persist-- 8 ence of vision, lag Lof Lphosphorescence fof 'the screen or both.

Although rtheA and B 'pulses `and the timing marksimaybemade f to appear on the 'cathoderay tracessimultaneously, it i is fusually preferredthat the A .and :B 'pulses only appear on the fswee-p traces .during .the 'alignment .step and'that only the timing marks :appear during the time lreading step. The following description assumesthat the .latter is accon'iplished by means of `suitable switching described'hereinafter. .After the pulses A and `B `have been aligned, the operator moves a switch |26 (Fig. 1) from its alignmentlposition to a time .reading position. The timing marker .pulsesnow appear on the sweep traces as shown in Figs. 3 Vand-'iand by counting certain of 'these timingma'rksgthe desired time difference between the pulsesA and B can be obtained. The number of T1000 ./1.. is. units in this time difference may lbe determined, for Vexample, :by counting the number Vof "1000 e. Vstiming markers on the trace ab which lie between the v20,000 fr. s. marker and the right end of the trace cd. Or it may be determined by counting from the 20,000 fr. s. marker to the end `b of the trace ab. In the example illustrated, the count is 4000 e. s. plus a fractional .1000 it. fs. interval. The additional number of microseconds in the time difference to be determined can be estimated'roughlyby the frac- .tional 10'00i. s. Spacing that remains Vbetween the last of these marks on the vtrace-'aib and the right end'ofrthe sweep cd (or theendof thesweep ab, as thefcasemay be) butin practiceiit'is 'determined precisely by counting t. s. timing markers, e. s. timing markers and-estimating the units ,at the expandedleft ends of the sweeps. This last feature will be described after ya more complete vdiscussion of the circuit.

From `the rforegoing discussion, Yit will be apparent that the amount that the starting time t .ofjgraph G (Figs. 4 and 5) .has to be shifted from some predetermined position such as the center of vv20,000 p.. s. position in'order to bring the pulse B intocoincidence with the pulse A is a measure of the time difference between the pulses A and B or, in the examplementioned, it is a measure of the amount vthat a B vpulse is away from the .mid-point of the repetition period of the A pulses.

General description of cathode-ray trace producing circuits 'The circuit for obtaining Vthe operation described in connection with Figs. 3, 4 and 5 will first Abe described generally with reference tothe block-diagram of .Fig- 1 and the graphs of Fig. 5.

lReferring to Figs. 1 and 5, the Eccles-Jordan ,oscillator 65 is triggered bythe 20,000 l. s. pulses supplied -over the conductor VEl to produce rectangular voltage wavesC tand C' of opposite polarity which are differentiated byvd-ifferentiating circuits Liiiv and 10:1 to produce the wavesD and D., respectively. The positive pulse portions of the waveD trigger the vmultivibrator IUI toproduce the rectangular wave IE. The timing-of the backed-goof the narrow pulse portion of the wave Eis adjustable -by means of the knob |202', this timing of vthe backk edge'controlling the starting time -t-of the second sweep portion of the deflecting `wavefG as will 'soon be apparent. The multi- V.vibrator Illl may be v-of the -well known type described in British Patent 456,840 to White and in the A...I.A.E. E. for June 1940, pp. 40 to 119.

The-.rectangular wave Efrom the multivibrator .LIM .is'passed-through a differentiating circuit |08 .to produce the' waveY F which is .supplied over `a amplifier I I3 to the horizontal deflecting plates 4||4 of the cathode-ray indicator tube ||6. The

circuit III will be described in detail hereinafter with reference to Fig. 6. y

From the foregoing description and from a reference to the sweep separation wave K (Figl 5) which is applied to the lower vertical deiiecting plate I I1, it will be apparent how the sweep traces 'ab and cd are obtained.

Referring again to the block and circuit diagrams of Figs. 1 and 6, the vertical deflection or trace separatio-n wave K is produced by supplying-the wave C from the E-J oscillator 65 over a conductor I I8 to a mixing and clipping tube I I9 where the wave C and a Wave H are added and reversed to produce the wave I (Fig. 5) which is then clipped and the clipped wave reversed to produce the wave J. The Wave H is supplied from the multivibrator I 0| over a conductor |2| and through a polarity reversing tube |22 to the mixing and clipping tube I I9. The output of the tube I |9 is the wave J which, when reversed in polarity by a tube |23, is the desired trace separation wave K. The wave K is supplied over a conductor |24 to the lower vertical deflecting plate |I1.

To make a time measurement, the operator throws a switch |26 first to a pulse alignment position for aligning the pulses A and B received from a pair of ground transmitters, and then throws it to a time marker reading position (the switch position illustrated in Fig. 1), to count time marker pulses. In the align position of switch |26, a radio receiver 21 supplies 'the A and B pulses of a pair of ground stations over a conductor |28 to the upper vertical deilecting plate ||1. The receiver |21 is tuned to the carrier wave frequency common to all the transmitter ground stations of the navigation system, station selection being by means of the different pulse repetition rates for different pairs of stations as previously described.

In the time marker read" position of switch |26, the time marker pulses of u. s., 100 a. s., 1000 fr. s. and 20,000 a. s. repetition periods are supplied from the mixer tube |29 over a conductor |3I to the upper vertical deflecting plate ||1, .the A and B pulses no longer being applied to the cathode ray tube ||6.

Detailed description of Figure 6 The detailed description of Fig. 6 will now be given. Referring to Fig. 6, the multivibrator |0| comprises two triodes |32 and |33 which are connected to form a cathode-coupled multivibrator. The pulses D from the differentiating circuit |06 are applied to the grid of the triode |32, this grid having an adjustable positive bias applied thereto from a potentiometer resistor |02 through a grid resistor |34. This bias is adjusted by means of the control knob |02 for adjusting the time of occurrenceof the back edge of the narrow multivibrator pulse of. the Wave E. The diierentiating circuit |06 comprises a small coupling capacitor |36 and the grid resistor |34,

The differentiating circuit |01 applies the pulses D' to the grid of a triode IIUA that forms part of the mixing circuit IIO. The differentiating circuit |01 comprises a small coupling capacitor |31 and a grid resistor I 38. The dilerentiating circuit |08 applies the pulses F to the grid of a triode I |0B forming the other part of the mixer III), the tubes II 0A and I.|0B having a common cathode resistor |39 across which the mixed signals D' and F appear. The diiferentiating circuit |08 comprises a small coupling capacitor |4| and a grid resistor |42.

The mixed signals D and F preferably are applied through a clipper'diode |43 to keep the amplitude 'of the positive pulses a constant value. The vclipped pulses are applied to the grids of a pair of triodes 44 and |46.

The triodes |44 and |46 have wave shaping -cathode circuits |41 and |48, respectively. The

circuit |41 comprises a cathode resistor |49 and a capacitor |5| in parallel therewith. The circuit |48 comprises a cathode resistor |52 and a capacitor |53 in parallel therewith.

Upon the occurrence of either a pulse D' or a pulse F, the tubes |44 and 46, which are normallybiased to cut-off, conduct anode current to charge the capacitors 5| and |53, respectively. This chargingof |5| Vand |53 is practically instantaneous. At the termination of the pulse D' or F, the'capacitors' |5I andv |53 discharge at a rate that is slow compared with the charging rate and at a. rate that is determined by the time constants of the circuits |41 and |48, these time constants differing from each other. Thus, as illustrated in Fig. '1, across the circuits |41 and |48 there are produced the voltage waves a: and y, respectively, which are to-be combined with a third voltage wave e to produce the desired logarithmic defiecting wave G.

4The wave e is obtained by applying the pulses D and F from the cathode resistor |39 over a conductor |54 to the grid of a tubeV |56 which includes a wave shaping network |51 in its cathode circuit. The network |51V comprises a `high impedance resistor |58, the lowerportion of a biasing resistor |59, and a capacitor |6I. As in the wave shaping circuits for producing the waves :1: and y, the capacitor |6I is charged rapidly upon application of a pulse to the tube |56. To produce wave z,however, the discharge of capacitor I 6| is made slow enough by proper adjustment of the time constant of network |51 so that it (unlike capacitors |5| and |53) has not discharged completely by the time the next pulse D' or F occurs. The wave z is applied to a cathode follower tube |60.

It will be seen that the effect of adding the Waves r and y to the Wave z is to greatly increase y and z are added by supplying them through leads |62, |63 and |64 to the input circuit of the deecting-wave amplifier II3. The leads |63 and |64 preferably include high impedance resistors |66 and |61 shunted by capacitors |68 and |69, respectively, for obtaining undistorted addition of the several waves. It Will be apparent that the deflecting wave G may be shaped as desired for different scale expansions by changing the time constants of one or more of the circuits |41, |48 and |51. Y

In Fig. 6, circuit values have been indicated in ohms, thousands of ohms, megohms, microfarads and micro-microfarads merely by way of example. Y

lil'

It should be understood? that the invention is not limited to the use ofl the-speci'c logarithmic deecting wave circuit described above and; in fact, is not limitedtothe use of a delecting wave logarithmic in wave form as; an exponential shaped deflecting wave may-be used.

Differential. gain control. circuit A differential gaincontrolcircuit; for thev re'.- ceiver |21y preferably isprovided as indicated in Fig. 1 for the purpose of keeping thearnplitudes ofthe A. and B pulses substantially alike atthe receiver output; thusk facilitating the, A and B pulse alignment. The gain control circuit, includes a potentiometer resistor- Hilf. to. theopposite ends of which the waves.JandKareeapplcd An adjustable tap |J2fmay'be movedtoeither side ofthe centerof'resistor llzl to decrease the gain of. thereceiver. l2? during either'thereceptionof the pulse A orY the pulseiB; As thiscrcuitxforms no part of the present invention no: detailed description will be given. Itzmay be-,noted1that',the diierential. gain control; circuitmay be the-.same as-that describedinzapplicationSerialNo. 560,648, led October 27, 19414,.andfentitled.Radionavigaftionsystem, in the nam-e of. Georgenl-lulst, Jr.

Measurement'of fractional'l' a. szrmterval' Under. the headin gcathode-Iay 'tracea andz tim.- ingmarker presentationithas beenexplainedyvith reference to Fig.. 3 howv the number of; 10.00 les. intervalsin the time differencemeasurement .may be. found'v by counting 100.0 p.. s; markers on the trace ab from the120,000 p.. s. marker tothe-right endof eitherfthetracezab-or the-.trace cd..

Instead of estimating the remainingfractional 1000's. s. interval, `itis foundfaccuratelyby counting 100 a. s. and 10u. s. markersonthe-.expanded part of.' the lower. trace cdaszfollows: The 100/i. s. marks on thetrace cdzfrom a 100041.; s; mark on the.` trace cd to the first 1000 n. s; mark tothe right on the upper tracey ab. are counted Next the number-of ,u..s. marks-on the trace cdfrcm a- 10.0. fr. s. mark" on thetrace cd: tothe iirst 100 p.. s. mark to thezright4 onthe upper traceY ab are counted. The remaining. interval from the last 1.0 p.. s: mark counted to said 100m. s..mark.on.the upper trace is-.estimatedin 1 p.. s..int'erva'l's. In the exampleshownin Fig. 3, thisprocedure gives 833 fr. s'. Sincethere are-four 1000 u. s. markers between the 20,000`u. s. mark. andthe end dof the traced cd; the total reading is.4833p..s.

We claimas our invention:

1. In a navigation system,whereinfperiodically recurring radio pulses are radiated! from A` and Bground stations as A and Bpulses, respectively, with the Bv pulses occurring at a predetermined time following the mid-point of the periodof'the A pulses, the method of measuring the time interVal-between the A and B pulses at a point remote from said ground stations' which comprises receiving said A and B pulses at said point, pro ducing successively pairs of sequentially occur-ring deecting waves having decreasing slope from at least near the start of-f the wave which are identical throughout their useful deflecting portions, deflecting a-cathode ray by said' Waves to produce two cathode ray traces, causingsaid A and B pulses to appear on said two traces with the B pulse-on the trace that is producedl by thesecond of said pair of deflecting waves, adjusting the starting time of the second ofi said pairofdefleeting waves until it is such thatl said A and B pulses are aligned, producing groups' ofv timing pulses with each group having a different'repeti'- tion period, and causing said timing pulses toapipeary as timing marks on said traces whereby said time interval between the A and B. pulses maybe found by. counting certain of the longer intervalA timing marks on the less expanded' end of one of the traces and by counting certain of the shorter interval timing marks on the., more expanded end of one of said traces.

2.. In a navigation system wherein periodically recurring radio pulses are radiated from. A, and Bground stations as A and B pulses, respectively,v with the B pulses occurring at a predetermined time following the mid-point of theperiod'o the A pulses.` the method of measuring the time-.intervalbetween the.A-an.d B puisespatapointre.- mote.. from said ground'. stationsV which. comprises receiving said A and Bpulses at said point, pro.b ducing successively pairs of sequentiallyfocurring deflecting'waves having decreasingslope from at least near the. start of the wave which are identical throughout their-useful deflectmg portions, deflectinga cathode ray by said waves to produce two-parallel adjacent. cathode ray traces, causing said Av andB pulses to appear on said twotraces with: the B pulse; on the trace that is produced by thesecondof saidipair of deectingwaveaadjusting the starting time of the second ofv said pair of' deflecting waves until it is such that said A and B pulses are aligned, producing groupsof timingA pulses including groups having repetition periods of 1000;. s. 100/i. s. and 10p. s. andcausingjsaidf timing pulses to appear as timing marks onsaid tracesl whereby said time interval between theA and B-pulses may be found by counting-cer.- tain of the 1000 u. s. timing marks on the less expanded/end of one of the traces andby counting certain of the u. s. and 10 a. S. timing marks onthemoreexpanded end of one'of said traces.

3. In a navigation system wherein periodically recurring radio pulses are radiated from.A and B- ground stations as Aand'B pulses, respectively, with the.B'pu1ses-occurring at a predetermined time following the mid-point of the period of the A pulses, means for measuring the time inter-val between the A- and B pulses at a point remote from-said ground stations comprising means for receiving said A and Bfpulses at said point, means for producing successively pairs ofsequentially occurring; deflecting waves having decreasing slope from` at least near the start of thewave Which are identical. throughout their useful deecting portions, means for deflecting a cathode ray by said waves to produce two cathodey ray traces, means for causing said A and B pulsesto appear-'onsaid two traces with the B'pulse on the trace .that is produced by the second of said pair ofdeectingrwaves, means for adjustingthe starting time of the second of said pairof' deiiecting waves until it issuch that said A and B pulses are aligned, means for producing groups of timing pulses with each group having a different repetition period, and means for causing said timingY pulses to appear as timing marks on said traces whereby said time interval between the A andB pulses may be found by counting certain of the longer interval timing marks on the less expanded end of one of the traces and by counting certain of the shorter interval timing marks on the more expanded end of one of said traces.

4. The method of measuring the time relation of a group of periodically recurring received A pulses with respect to a group of periodically -received B pulses where both groups of pulses have the same repetition period and where each B'pulse occurs following the :mid-pointu of the A pulse period, said method comprising the steps of producing two successive cathode-ray deflecting waves starting at the same voltage level and each of identical slope and having a total repetition period equal to that of said groups of pulses, the slope of each wave being of decreasing steepness from at least near the start of the wave to the end of the Wave, producing groups of timing pulses each group having va dilerent repetition period and having a xed time relation to the start and nish of the cycle of said two defiecting Waves, causing each of said deiiecting waves to produce a cathode-ray trace the first portion of which is expanded and causing said A and B received pulses to appear on the expanded portions of the cathode-ray traces produced by the rst and second of said delecting waves, respectively, changing the start of said second deiiecting wave of the deflecting wave cycle with respect to a predetermined point in said cycleuntil the received pulses on said traces are in alignment or coincidence, causing said groups of timing pulses to produce timing marks on said traces whereby from the resulting timing marks on the less expanded portion of a trace an operator may determine within a fraction ofthe longest of said repetition periods to be counted the amount that the start of said second wave is shifted in time with respect to said predetermined point in said deecting wave cycle, and counting from a certain timing mark on the expanded portion f one of said traces to a corresponding timing mark on the expanded portion of the other trace to determine said fractional repetition period, said certain timing mark and said corresponding timing mark being produced by'two of said shorter-repetitionperiod pulses one of which follows the other along said traces in time sequence.

5. In a navigation system, receiving apparatus for measuring the time relation of periodically recurring received pulses A transmitted from a ground station A with respect to periodically recurring received pulses B transmitted from a second ground station B where the A and B pulses are transmitted with the B pulse occurring a predetermined time following the mid-point of the A pulse period, said apparatus comprising means for producing two successive cathode-ray deiiecting waves starting at the same voltage level and each of identical slope and having a total repetition period equal to that of said A and B pulses, the slope of each wave being of decreasing steep* ness from at least near the start of the wave to the end of the wave, means for producing groups of timing pulses each group having a dierent repetition period and having a fixed time relation to the start and iinish of the cycle of said two deecting waves, means for causing each of said deflecting waves to produce a cathode-ray trace the iirst portion of which is expanded and means for causing the A and B pulses toappear on the expanded portions of the cathode ray traces produced by the first and second of said two deiiecting waves, respectively, means for changing the start of said second deecting wave of the deflecting wave cycle with respect to a predetermined point in said cycle until the received pulses on said traces are in alignment or coincidence, means for causing said groups of timing pulses to produce timing marks on said traces whereby from the resulting longer repetition period timing marks on one of said traces an operator may determine within a certain fraction of saidlonger period the amount that the start of said second Wave is shifted in time with respect to said predetermined point in said deecting wave cycle, and whereby the operator may count shorter repetition period timing marks on one of said traces from a certain timing mark on the expanded portion of said one trace to a corresponding timing mark on the expanded portion of the other trace to determine said fractional repetition period, said certain timing mark and said corresponding timing mark being produced by two of the shorter-repetition-period pulses one of which follows the other along said traces in time sequence.

6. The method of measuring the time relation of a group of periodically recurring received A pulses with respect to a group of periodically received B pulses where both groups of pulses have the same repetition period and where the B pulse occurs following the mid-point of the period of the A pulses, said method comprising the steps of producing two successive cathode-ray deflectingr waves starting at the same voltage level and each of identical slope and having a total repetition period equal to that of said groups of pulses, the slope of each wave being of decreasing steepness from at least near the start of the wave to the end of the wave, producing groups of timing pulses having repetition periods of 20,000 y. s., 1000 p. s., p. s. and l0 p. s., respectively, each group having a Xed time relation to the start and finish of the cycle of said two deflecting waves, causing each of said deflecting waves to produce a cathode-ray trace the first portion of which is expanded and causing said received A and B pulses to appear on the cathode-ray traces produced by the rst and the second of said deflecting waves, respectively, on their expanded portions, delaying the start of the second deflecting wave of said deflecting cycle with respect to the mid-point of said deflectingwave cycle until the received pulses on said traces are in alignment or coincidence, causing said groups of timing pulses to produce timing marks on said traces, counting the resulting 1000 p. s. timing marks on the less expanded portion of the longer trace from the 20,000 p. s. mark to the end of one of the traces to determine within a certain fraction of the 1000 e. s. repetition period the amount that the start of said second wave is shifted in time with respect to said mid-point of the full deflecting wave cycle, counting 100 p. s. marks from a 1000 p. s. timing mark on the expanded portion of the longer of said traces to the next preceding 1000 p. s. timing mark on the shorter trace t0 determine said fractional 1000 p. s. repetition period to the nearest 100 n. s. interval and counting l0 p. s. marks from a 100 p. s. timing mark on the expanded portion of said longer trace to the next preceding 100 a. s. timing mark on said shorter trace to determine the fractional 100 ,u. s. interval to the nearest 10 n. s interval.

GEORGE D. HULST, JR. EARL SCHOENFELD. GARRARD MOUNTJOY. 

